Electrical heaters are often used in petrochemical processing plants. Typically, the heater has a tank with an inlet and an outlet for the circulation of a fluid to be heated. Heater tubes are installed within the tank, each heater tube containing an electrical resistance wire within a powder insulation. Each heater tube has one or two ends that are welded to an exterior side of a header plate. The header plate bolts to the tank, and the exposed ends of the electrical resistance wires extend into a housing for connection to electrical power wiring. The housing may be a cylindrical wall that is welded to the header plate. The housing has an outer end with a removable closure plate.
If a heater tube becomes faulty and needs to be repaired or replaced, a worker normally removes the header plate and assembly of heater tubes from the tank. The worker removes the closure cap from the housing, which provides access to the welds holding the heater tubes to the header plate. Even if the particular tube to be removed is located in the center of an extensive bundle of tubes, the worker has access to its weld to the header plate. The worker can thus remove the particular heater tube by cutting and grinding away the weld.
Some heaters have standoff housings, which locate the housings a selected distance from the header plate. In one prior art type, conductor rods are welded or brazed to the pins of the electrical resistance wires to form extensions. An extension or standoff tube is placed over the conductor rod, and the lower end of the standoff tube is abutted against one of the heater tube ends and welded to the exterior side of the header plate. The standoff housing has a base plate with holes for each of the standoff tubes, and the housing is placed over the free ends of the standoff tubes. A worker welds each standoff tube to the interior side of the base of the standoff housing.
In another type, the heater tubes have lengths selected so that they protrude beyond the header plate for the standoff distance. An intermediate portion of each heater tube is welded to the header plate, and the end of each heater tube is welded to the base plate of the standoff housing.
In both types of standoff housing arrangements mentioned above, the initial assembly of heater tubes or standoff tubes can be made by working from the inside of the bundle outward. Removing a particular element after assembly can be a problem if such element is located within the central portion of a bundle of tubes. In the first type, the open ends of the heater tubes are not accessible because extension tubes have been abutted and welded to the header plate. The open ends of the extension tubes are accessible in the offset housing, but grinding away the weld of one of the extension tubes does not provide any access to the open end of the heater tube. Rather a worker may have to remove many heater elements not otherwise selected for replacement merely to provide access.
In the second type, although the heater tube open ends are accessible by removing the closure plate in the offset housing, the heater tubes are also welded to the header plate. That weld would not be accessible to a worker if the particular heater tube were located in a central area of the bundle.